Real-Space Observation of Skyrmionium in a Ferromagnet-Magnetic Topological Insulator Heterostructure.

The combination of topological insulators, that is, bulk insulators with gapless, topologically protected surface states, with magnetic order is a love-hate relationship that can unlock new quantum states and exotic physical phenomena, such as the quantum anomalous Hall effect and axion electrodynamics. Moreover, the unusual coupling between topological insulators and ferromagnets can also result in the formation of topological spin textures in the ferromagnetic layer. Skyrmions are topologically protected magnetization swirls that are promising candidates for spintronics memory carriers. Here, we report on the observation of skyrmionium in thin ferromagnetic films coupled to a magnetic topological insulator. The occurrence of skyrmionium, which appears as a soliton composed of two skyrmions with opposite winding numbers, is tied to the ferromagnetic state of the topological insulator. Our work presents a new combination of two important classes of topological materials and may open the door to new topologically inspired information-storage concepts in the future.